(1) Field of the Invention
The present application relates to a surface-mountable over-current protection device, and more particularly to a surface-mountable over-current protection device with superior resistance repeatability.
(2) Description of the Related Art
Because the resistance of conductive composite materials having positive temperature coefficient (PTC) characteristic is very sensitive to temperature variation, it can be used as the material for current sensing devices, and has been widely applied to over-current protection devices or circuit devices. The resistance of the PTC conductive composite material remains extremely low at normal temperature, so that the circuit or cell can operate normally. However, when an over-current or an over-temperature event occurs in the circuit or cell, the resistance instantaneously increases to a high resistance state (e.g., at least 102Ω), so as to suppress over-current and protect the cell or the circuit device.
A known PTC material usually uses carbon black as conductive filler which is evenly dispersed in crystalline polymer. In this crystalline structure, the carbon black particles are usually aligned at grain boundaries and are arranged closely. Accordingly, current can flow through the insulating crystalline polymer through such “carbon black chains.” At normal temperatures such as room temperature, numerous carbon chains exist in the polymer and constitute conductive paths.
When the current make the device temperature increase to a temperature exceeding the phase transition temperature such as the melting point of the polymer, the polymer expands to change the crystalline state to amorphous state. As such, the carbon chains are broken and thus current is not allowed to pass therethrough, and as a consequence the resistance increases tremendously. The phenomenon of instant increase of resistance is the so-called “trip.”
When the temperature decreases to below the phase transition temperature, the polymer is re-crystallized and the carbon black chains are rebuilt. However, the polymer cannot be fully recovered after expansion so that the carbon chains cannot sustain original conductivity and the resistance cannot return to initial low resistance. After tripping many times, the resistance may increase significantly, resulting in poor resistance recovery or poor resistance repeatability.